1. Field of the Invention
This invention relates to a transmitter/receiver apparatus for use for the TDMA/TDD (time division multiple access/time division duplex) system.
2. Description of the Prior Art
In a transmitter/receiver apparatus of the TDMA/TDD system which is employed in a tele-point system and so forth, a transmission frequency is equal to a reception frequency, and one frame is constituted, for example, as shown in FIG. 2, from transmission slots T1 to T4 and reception slots R1 to R4. The slots T1 and R1, T2 and R2, T3 and R3, and T4 and R4 are each used in pair.
Since such transmission and reception are performed, a transmitting circuit and a receiving circuit of a transmitter/receiving circuit of the TDMA/TDD system are constructed, for example, in such a manner as shown in FIG. 3.
Referring to FIG. 3, a transmitter/receiver apparatus includes a transmitting circuit 10 and a receiving circuit 20. In the transmitting circuit 10, an aural signal Sa is supplied to a transmission processing circuit 11, at which processing of the aural signal Sa for transmission of the TDMA/TDD system is performed. The thus processed aural signal Sa is extracted, for example, for each transmission slot T1.
Such signal Sa is supplied to an FM (frequency modulation) modulating circuit 12 constituted from a PLL (phase-locked loop), at which it is converted into an FM intermediate frequency signal Sit of a predetermined intermediate frequency fit, for example, of fit=300 MHz. The FM intermediate frequency signal Sit thus produced is supplied to a switching circuit 13, which is turned on for the period of each transmission slot T1 so that the signal Sit is outputted from the switching circuit 13 for the period of each transmission slot T1.
The signal Sit thus outputted is supplied to a mixer circuit 14 while an oscillation signal So having a frequency fo is outputted from an oscillating circuit 31 for selection of a transmission/reception channel and supplied to the mixer circuit 14 so that the signal Sit is frequency converted into an FM signal St of another frequency ft. It is to be noted that, in such frequency conversion. EQU ft=fit+fo
and for example, ft=2.6 GHz.
The FM signal St is supplied to an antenna 34 along a signal line including a band-pass filter 15, a power amplifier 16, a high frequency switching circuit 32 and a low-pass filter 33 so that it may be transmitted from the antenna 34.
On the other hand, in the receiving circuit 20, an FM signal Sr transmitted thereto for the period of a reception slot R1 is received by an antenna 34. It is to be noted that the frequency fr of the received FM signal Sr is equal to the frequency ft of the transmission frequency St.
Then, the received signal Sr is supplied to a first mixer circuit 23 by way of a signal line including the filter 33, the switching circuit 32, a band-pass filter 21 and a high frequency amplifier 22. The oscillation signal So from the oscillating circuit 31 is supplied also to the mixer circuit 23 so that the signal Sr is frequency converted into a first intermediate frequency signal Sir. It is to be noted that, in this instance, since fr=ft, the signal Sir has an intermediate frequency fir equal to the frequency fit of the signal Sit.
Then, the signal Sir is supplied to a second mixer circuit 25 by way of a first intermediate frequency amplifier 24 while a second local oscillation signal is supplied from a second local oscillating circuit 26 to the mixer circuit 25 so that the signal Sir is frequency converted into a second intermediate frequency signal having an intermediate frequency of, for example, 10.7 MHz. A signal obtained by such frequency conversion is supplied by way of a second intermediate frequency amplifier 27 to an FM demodulating circuit 28, at which the signal is demodulated into an aural signal Sb for each reception slot R1. The aural signal Sb is supplied to a reception processing circuit 29, at which processing for reception of the TDMA/TDD system to extract the original aural signal Sb is performed.
By the way, since the FM modulating circuit 12 is commonly constituted from a PLL, if power is supplied to it only for the period of each transmission slot to put it into an operative condition, then it cannot perform an ordinary FM modulating operation for the period of each transmission slot due to a rising characteristic of such PLL.
Therefore, power is supplied to the FM modulating circuit (PLL) 12 also for the period of each reception slot. If power is supplied in this manner, the signals Sit and St are outputted also for the period of each slot. Accordingly, the switching circuit 13 is turned off and the power supply to the circuits 15 and 16 is interrupted for the period of each reception slot.
However, even if the switching circuit 13 is turned off and the power supply to the circuits 15 and 16 is interrupted for the period of each reception slot in this manner, the FM modulating circuit 12, that is, the PLL itself, continues to oscillate with the oscillation frequency fit which is equal to the first intermediate frequency fir of the receiving circuit 20.
Consequently, the oscillation signal Sit of the modulating circuit 12 will plunge into the circuits 23, 24 and 25 of the receiving circuit 20 to cause a disturbance to reception thereby to deteriorate a substantial reception sensitivity of the receiving circuit 20.
In this instance, if the oscillation frequency fo of the oscillating circuit 31 is varied between transmission and reception, then fir.noteq.fit, and consequently, a direct disturbance can be eliminated.
Meanwhile, in order to allow a change of a channel, also the oscillation circuit 31 is commonly constituted from a PLL. Further, since the interval between transmission and reception is several milliseconds, if the difference of the oscillation frequency fo between transmission and reception is great, then a normal oscillation frequency cannot be obtained due to a responding characteristic of a PLL.
However, if the difference of the oscillation frequency fo between transmission and reception is set to a comparatively small value corresponding to one or two channels, then when the oscillation signal Sit of the modulating circuit 12 plunges into the circuits 23, 24 and 25 of the receiving circuit 20, where the frequency characteristic of the intermediate frequency amplifier 24 corresponds to one or two channels or so, the signal Sit cannot be separated, and consequently, a disturbance to an adjacent channel may be caused or a disturbance of cross modulation may be caused with another signal of another frequency received by the antenna 34.
Further, in case the transmitting/receiving frequency or frequencies belong to a quasi-microwave band as described hereinabove, the frequency fir belongs to a VHF or UHF band in most cases, and accordingly, it is difficult also from the point of view of mounting to prevent a roundabout of the signal Sit from the FM modulating circuit 12.